In: Biology
Let’s say a cell has arrested in the cell cycle due to a chromosome being temporarily unable to attach to the microtubules of the mitotic spindle.
(a) At what stage of the cell cycle is the cell arrested? [ Select ] ["Prophase", "Metaphase", "Prometaphase", "Anaphase"]
(b) For each of the factors listed below, decide if the factor would be present in such a cell.
p21 [ Select ] ["PRESENT", "NOT PRESENT"]
M-cyclin [ Select ] ["PRESENT", "NOT PRESENT"]
Cdk [ Select ] ["PRESENT", "NOT PRESENT"]
Securin [ Select ] ["PRESENT", "NOT PRESENT"]
Separase [ Select ] ["PRESENT", "NOT PRESENT"]
APC/C [ Select ] ["PRESENT", "NOT PRESENT"]
G1 cyclin [ Select ] ["PRESENT", "NOT PRESENT"]
(c) Now let’s say that the chromosome finally does attach correctly to the spindle, so that all of the chromosomes have now achieved proper biorientation and congression to the metaphase plate. Make the correct selections to fill in the blanks regarding the molecular events that lead to sister chromatid separation:
Proper [ Select ] ["Motor protein activity", "Balance", "Ndc80 phosphorylation", "Tension"] on all of the kinetochores causes the spindle assembly checkpoint protein, [ Select ] ["Cdc20", "Aurora B", "Mad2", "Ndc80"] , to be inactivated. [ Select ] ["APC/C", "APC/C-Cdh1", "APC/C-Cdc20", "SCF"] can now ubiquitinate [ Select ] ["separase", "securin", "M-cyclin", "S-cyclin"] , which is then degraded by the [ Select ] ["Lysosome", "Acid hydrolases", "ECM protease", "Proteasome"] , thereby releasing active [ Select ] ["Microtubules", "Motor proteins", "Separase", "Cohesin"] , which cleaves the [ Select ] ["Cohesin", "Condensin", "Microtubules", "Ndc80 proteins"] that was holding the sister chromatids together.
The stage at which the cycle is arrested is the prometaphase. It is this phase, of mitosis that is characterised by the attachment of chromosomes (in form of kinetochores) to attach to the kinetochore microtubules.
For the factors give:
p21 is protein involved in association to the p53 that are involved in the arresting of cell cyle in the in cause of DNA damage cause in the S phase. Hence, these proteins would be produced during the S phase and are not present in a cell undergoing M phase.
M-cyclin is protein produced at the start of the metaphase and governs the spindle formation and the alignment of chromosomes on the mitotic spindle. Since, the chromosomes are being temporarily unable to attach these, proteins are pesent but low in concentration.
Cdk (Cyclin dependent kinase) are proteins acting as checkpoints for the different phases of the cell cycle and hence are present in the cell.
Securin is protein that secreted at the onset of anaphase, where it causes highly synchronous separation of chromatids, and hence are not yet produced, and not present in the cell.
Separase, in association with securin degrades the cohesins between the chromatids that lead to their separation in the anaphase, hence these are not produced or present.
APC/C Anaphase promoing complex is a ubiquitin ligase that degrades the securing after the separation of sister chromatids, hence this is also not present in the cell.
G1 cyclin is a protein as the checkpoint to enter the S phase. This is not related to the M phase and hence, not present in the cell.
Proper tension on all of the kinetochores causes the spindle assembly checkpoint protein Aurora B to be inactivaed. APC/C-Cdh1 can now ubiquitinate securin which is then degraded by the proteasome, thereby releasing active separase which cleaves the cohesin that was holding the sister chromatids together.